Mobile communications have changed people's lives and people have also new higher requirements for mobile communication networks. Mobile networks operator needs to provide better coverage, higher user throughput (especially for cell edge users), lower transmission latencies and longer battery life of mobile station. These are also the target of IMT-Advanced (International Mobile Technology-Advanced), and several standardization organizations have launched new studies for next generation mobile communications, e.g. IEEE (The Institute of Electrical and Electronics Engineers) 802.16m and LTE-Advanced(Long Term Evolution-Advanced), etc.
As a cost-efficient way to extend coverage and enhance user throughput (especially for cell-edge users), multi-hop relay technique has been employed in the WiMAX (Worldwide Interoperability for Microwave Access) system and LTE-Advanced.
As for longer battery life, on the one hand, advanced design methods and new material should be employed to increase battery capacity; on the other hand, with the help of mobile communication network, the negotiation mechanism between mobile station (MS) and network is utilized, for example, the introduction of sleep mode of the mobile station can reduce the awake time of the mobile station, thus saving the power consumption of the mobile station. However, sleep mode has some negative influence on the latency-sensitive service, that is, the real-time service that requires low latency, such as voice communication service, etc. Therefore, how to trade off between power-saving of the battery and real time property with a negligible overhead in multi-hop relay system is a technical problem which will be resolved in present invention.
Sleep mode means the negotiation between the MS and network, the base station only schedules the data transmission at a special interval, named “listening window”; and the MS keeps the sleeping status during the other intervals, named “sleeping interval”.
Considering that multi-hop relay system comprises multiple links, if some errors occur at any link on the path between BS to MS, the rescheduling and retransmission will he implemented, and then “listening interval” may be missed by the MS. The current solution is to wait for the next “listening interval” to transmit data to MS; as shown in FIG. 1, the detailed procedures of waiting for the next “listening interval” to retransmit data can be described as follows.
As shown in FIG. 1, for example, the BS1 in multi-hop relay system adopts centralized scheduling and the data packets use the coding scheme of HARQ (Hybrid Automatic Repeat reQuest), the process flow when the relay station (RS) 2 detects wrong packets is described as follows. The BS 1 shown in FIG. 1 is located in vs ireless multi-hop relay network, the BS 1 can establish communication with the MS 3 via one or more RSs 2, and can also establish communication with the MS 3 directly.
At Frame N, as shown in step i of FIG. 1, the BS 1 schedules an initial transmission of HARQ packet on all the links between the BS 1 and MS which comprise not only relay link between the BS 1 and the RS 2 or between the RS 2 and the RS 2 but also access link between the BS 1 and MS 3 or between the RS 2 and the MS 3, that is, allocates communication resources such as time and frequency resources for all of network apparatuses on the relay link and access link, and sends the DL HARQ burst to the next hop RS 2; moreover, at the same frame BS 1 sends DL MAP comprising RS relay MAP message to the RS 2, and sends RS access MAP message to the RS 2, as shown in step i of FIG. 1.
Then, as shown in step ii of FIG. 1, the RS 2 verities whether the DL HARQ burst in the data frames sent to the MS 3 is wrong. Specifically, the RS 2 can check through CRC (Cyclic Redundancy Code).
Then, as shown in step iii of FIG. 1, if wrong, then the RS 2 modifies the RS DL MAP of itself and replaces the transport CID of the MS 3 with the management CID (connection ID) such as basic CID of itself, and sets the data subcarrier of the wrong data burst to be sent to the MS 3 null.
Then, in step iv, the RS 2 sends the modified DL MAP, null data subcarrier and corresponding pilot to the MS 3. Since the transport CID of the MS 3 is replaced by the management CID of RS, actually, the MS 3 does not know there is data sent to it.
Furthermore, the RS 2 sends NACK (Not Acknowledge) message to notify the transmission failure according to the retransmission mechanism configured by network. Then, the BS 1 will schedule retransmission.
The aforesaid description of the prior arts is explained with the example that the nearest RS 2 to the BS 1 detects transmission error. Certainly, the position of the RS 2 which detects error is not limited by aforesaid example, the RS 2 which detects error can be in the second hop, the third hop, etc. The RS in different positions processes the wrong packet in similar steps to aforesaid steps; therefore, it is no necessary to repeat again.
The scheduling of the BS 1 must guarantee that the retransmission of data packets reaches the MS 3 within the listening window of the MS 3, so the MS 3 will not lose data packets. In case that wrong packet does not occur, the scheduling of the BS 1 can guarantee that data packets reach the MS 3 within predetermined listening window; however, in case that wrong packet occurs, the BS 1 will reschedule data packets to make the retransmitted data packets reach the MS 3 within the next listening window of the predetermined listening window. Therefore, the current solutions will bring following disadvantages:                1.the MS 3 may receive data packets within the next listening window of the predetermined listening window according to the retransmission mechanism of the BS 1, therefore, this will cause longer latency and decrease the QoS (Quality of Service) of the real time service such as voice etc, and decrease the user experience;        2. since the null data subcarrier does not carry any useful information, so this wastes valuable wireless resource;        3. the intermediate RS 2 will store a plurality of unsuccessful transport packets because of relative long scheduling delay, thereby a plenty of storage is required and thus device cost of the RS 2 is increased.        